Sales pricing based on portion of purchase price

I will soon be a herbalist. I want to sell liquid herbs which I mix. I buy them by the 500ml or 2.5lt bottle and mix say 20ml of one, 60ml of another, 110ml of another, and 15ml of the last one. Each one of these is fine as a line in the sales docket.

But I’d like a price per ml automatically generated based on the purchase price plus my profit margin. I tried doing this on the demo over a year ago and couldn’t. I asked then how to do this but got zero replies. I just tried again but couldn’t seem to do what I wanted, with what is there. Can anyone help me with this?

I’m not a programmer but I’ve dabbled. Where do I start? Or is there someone who can do this for a fee?
(I thought maybe I could do something with the custom fields module)

Maybe ou can try Extraprice module from

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Yep downloaded and installed, gunna take a look to see it’ll do what I need
Thanks